It's a conflict of interest in that it's in the bars' (financial) interest to let people through.

It's not in the bars' financial interest to create a minimum drinking age via regulatory capture just so they can hire bouncers, or accept bribes or sell fake IDs or whatever. That's analogous to the insinuation I picked up upthread (if I read that comment correctly). Expecting industries to self-regulate until they show themselves incapable of doing so is pretty standard.

It is, but in practice it doesn't matter too much. In most (all?) jurisdictions in the US, bars can be fined pretty heavily or even lose their liquor license if they're caught serving people illegally, or if they even have people on the premises without their ID with them. Any extra revenue they might generate from letting in underage people isn't worth the risk of the consequences if caught.